A(1) Field of the Invention
The invention generally relates to a television system for transmitting television pictures in a digital form from a transmitter to a receiver. To limit the quantity of bits to be transmitted, each picture is subjected to a transform coding with different transforms, the transform to be performed being determined by the presence of motion effects between two consecutive fields of a frame.
The invention also relates to a motion detector for determining these motion effects.
It is to be noted that such a television system may be a television broadcasting system. In such a case a TV channel is used for transmitting the digitized television pictures.
Such a television system may alternatively be a video recorder, in which case the video tape is used for transmitting the digitized television signals.
A(2) Description of the Prior Art
As is generally known, a television picture is completely defined by three picture signals PS(1), PS(2), PS(3). These may be the three primary colour signals R, G, B, but alternatively a luminance signal Y and two colour difference signals CHR(1) and CHR(2) which are sometimes denoted by U and V, sometimes by I and Q, while many other designations are conventional.
For transmitting a television picture in a digital form, the television picture is considered to be a two-dimensional matrix of M rows (lines) each having N pixels (columns) and only the values associated with these M.times.N pixels of the three picture signals are transmitted to the receiver.
In a 625-line TV frame the visible part of the frame comprises 576 lines (rows) each having 720 pixels. If the associated luminance value for each pixel is represented by, for example an eight-bit code word, only the representation of all luminance values already requires approximately 3.10.sup.6 bits, i.e. a bit rate of approximately 75.10.sup.6 bits/sec in the case of 25 frames per second. This is inadmissibly high. The object envisaged by the researchers is to realise a bit rate of approximately 20 Mbit/sec, notably for recording digitized video signals on a magnetic tape or another storage medium.
To achieve this object, the series of television pictures is subjected to a transform operation (see for example References 1 and 2 in section C). The television picture to be coded is partitioned into a number of pixel blocks of E.times.E pixels each. A conventional value for E is eight so that a total number of 6480 pixel blocks is obtained with this partitioning. Each pixel block is subsequently considered to be a sum of E.sup.2 mutually orthogonal basic pictures B(i,k); i,k=1, . . . E each basic picture being formed from the transform of its respective pixel block and each basic picture having its own weighting factor y(i,k). These weighting factors are commonly referred to as coefficients. The indices i and k represent the spatial vertical and horizontal frequencies, respectively, in the basic picture (see Reference 3, chapter 10). Its significance will be apparent from the following. In the case of, for example, a discrete cosine transform, each pixel of the pixel block may assume a value between two extreme values. If these extreme values are white and black, respectively, each pixel may have a grey value. The associated grey values of the pixels succeeding one another in the vertical (or horizontal) direction are located on a curve which is cosine-shaped as a function of the location and is periodical with a period of (2E-1)/i. The reciprocal value of this period is referred to as the spatial frequency. In the case of, for example a Hadamard transform a pixel has either the one or the other value of two extreme values; in other words, it is either white or black. In this case i is a measure of the number of times of a jump from the one to the other extreme value, or conversely, in the series formed by the pixels succeeding one another in the vertical direction.
To transmit the coefficients at the lowest possible bit rate, they are subjected to an adaptive quantizing operation. A small step size is used for coefficients which are deemed to be significant, such as the dc coefficient y(1,1) and the step size increases as the significance of a coefficient decreases. The different coefficients are thus represented by different numbers of bits. It is to be noted that many of the less significant coefficients become zero (of course dependent on their value) due to said quantizing operation and thus are not transmitted at all or are not transmitted individually.
In this way a bit rate reduction which is by all means interesting is realised with only a slight loss of picture quality. As has been extensively described in References 1 and 5, the loss of picture quality is found to be small in practice only when the pictures to be transformed are stationary pictures. To visualize this, one may imagine an object in the picture, which object is bounded by a vertically extending line. If the object (and hence this line) is moved horizontally, the parts of this line in the even field will be offset with respect to the parts of this line in the odd field. Since each frame comprises these two fields in an interlaced form, the originally straight line will be meandering. By subjecting this frame to a transform operation, it will be necessary to transmit more coefficients of a higher vertical frequency for a satisfactory representation of this meandering line. This has a detrimental influence on the coding efficiency. To improve this, it has been proposed in Reference 1 to partition each pixel block into an even and an odd field block of E/2.times.E pixels each. The even field block comprises E/2 lines of the even field of the television picture and the odd field block comprises E/2 lines of the odd field of the television picture. Before subjecting a pixel block to a transform operation, it is first examined for motion effects. If these are not present, the entire pixel block is subjected to a transform operation which will be referred to as intraframe transform. If there are motion effects in the pixel block, the even and the odd field block are each and individually subjected to a transform operation. This will be referred to as intrafield transform.
Since such motion effects are not noticeable within a field, they will neither show if intrafield transform is used in such a case. In that case it is not necessary to consider more coefficients of higher vertical frequencies than would have been necessary if there had been a stationary picture which would have been subjected to an intraframe transform.
The decision whether the coefficients to be transmitted to the receiver are either those which are obtained from an intraframe transform or those which are obtained from an intrafield transform of the pixel block is made by a motion detector. An embodiment of such a detector is shown in References 1 and 5. In this embodiment a pixel block is subjected to an intraframe transform. More particularly, this is a modified two-dimensional Hadamard transform, but it may alternatively be any other modified transform, for example a Discrete Cosine Transform (DCT). The meandering line can now be represented by the basic picture having pictures representing the highest vertical and lowest horizontal frequency, i.e. by B(8,1) in the case of basic pictures derived from an 8.times.8 pixel block. This known motion detector now generates an indication of "motion" if the first coefficient y(8,0) associated with this basic picture B(8,1) and obtained by means of the modified two-dimensional Hadamard transform has a significant value which is, for example, larger than a predetermined reference value.
However, in practice this known motion detector regularly appears to take an erroneous decision in the sense that it indicates "motion", whereas there is no motion at all.